Bis(dialkylammonium)oxalates as accelerator activators in the vulcanization of elastomers



United States Patent or fice 3,487,056 Patented Dec. 30, 1969 ABSTRACTOF THE DISCLOSURE Bis(dialkylannnonium)oxalates and their hydrates,having the following structure:

where R is methyl or ethyl, and n is from 0 to about 3 are shown to beespecially active accelerator activators useful in the vulcanization ofelastomers.

It is desirable in the rubber industry to reduce the time andtemperature of vulcanization both for the purpose of producing a betterquality of elastomer and in order to increase the output of theelastomer from the vulcanization equipment. Toward this end, numerousvulcanization accelerators have been developed, and many of them are soactive that vulcanization is complete even at room temperature after afew days or after a few minutes at elevated temperatures. However,activation itself is not the only requirement necessary for a commercialaccelerator activator; since the chemical composition must haveproperties which are compatible with the rubber industry. For example,it is desired that compounds used in this capacity he crystallinesolids, easily handled and suitable for use in automatic weighingequipment which is widely used in the rubber industry. Furthermore, themelting ranges of the compounds used must be such that the materials areeasily dispersed in the dry mix of elastomer within the temperaturerange that is normally used for the addition of curing additives. Inaddition, the compounds must be easily and economically made, stable tostorage and shipment have a minimum effect on cure rate at processingtemperatures, and, of course, have the necessary activation to maketheir use of value. In accordance with the present invention, there isprovided as accelerator activators for elastomers, bis-dimethyl andbis-diethylammonium oxalates and their hydrates which meet the aboverequirements and which are, indeed, excellent accelerator activatorssuitable for commercial development.

Bis-oxalates useful in the invention are prepared readily from oxalicacid. The anhydrous materials are generally prepared in an isopropylalcohol solvent system, whereas the hydrates are prepared in aqueoussystems. The following examples illustrate the preparation of thesecompounds:

EXAMPLE 1 Anhydrous bis (dimethylammonium) oxalate At 510 C., 90 g. (2.0moles) of anhydrous dimethylamine was dissolved in 400 ml. of isopropylalcohol. A solution of 90 g. (1 mole) of oxalic acid in 400 ml. ofisopropyl alcohol was then prepared and added with stirring to thedimethylamine solution. The white precipitate which formed was filteredand washed with 100 ml. of

co1d. isopropyl alcohol, The product was dried 2 days in a vacuumdesiccator over anhydrous calcium chloride. The yield of whitecrystalline solid was 169.5 g. (94.2% M.P. 168-170 C;

Analysis.'Percent C=39.7, percent H=9.45, percent N'=15.32. Calcd:percent C=40.0, percent H=8.88, percent N=15.58.

EXAMPLE 2 Bis(dimethylammonium oxalate trihydrate Oxalic acid dihydrate(126 g.=1 mole) was gradually added to 226 g. (2 moles) of 40%dimethylamine during cooling in an ice bath to keep the temperaturebelow 60 C. The resulting solution did not precipitate crystals oncooling. Water was removed in a flash evaporator to C. at 30 mm. Theresulting water-white oil was poured onto a plate where it quicklysolidified to a white solid which weighed 191.5 g. and had a meltingrange of 55- C. This weight did not change on air drying for 3 days. Asmall portion of crude product was recrystallized from isopropyl alcoholto give material melting at 58- 74 C. Analysis indicated this to containand thus to correspond to the formula:

ensuring 64": .3H2O

Bis(dimethylammonium) oxalate sesquihydrate A 58.8% dimethylaminesolution (516 g.=4 moles) was cooled to 15 C. and 252 g. (2 moles) ofoxalic acid dihydrate was gradually added while the temperature wasmaintained at 15-20 C. by cooling in an ice bath. At the end of thisaddition, all of the oxalicacid had not reacted. The reaction mixturewas allowed to warm to room temperature to complete the reaction, aclear solution being obtained after a short time. Water was removed fromthis solution in a flash evaporator to 70 C. at 15 mm. to' give an oilwhich rapidly solidified on cooling to give a white solid. This Weighed400.5 g. and had a melting range of 56-138 C. Analysis indicated thismaterial to contain and hence contain 1.46 moles of water of hydrationwhich closely corresponds to a sesquihydrate composition.

EXAMPLE 4 Hydrate of bis(diethylammonium) oxalate A solution of 156 g.(2 moles) of diethylamine in cc. of water was prepared. To this duringstirring and cooling in an ice bath, 126 g. (1 mole) of oxalic aciddihydrate was added. The temperature of the reaction solution was keptbelow 60 C. during this addition. 0n cooling the clear reaction solutionto 20 C., White solid precipitated. This was filtered 0E and air driedto give 117.5 g. of product having a melting range of 93120 C. Thisfiltrate was stripped of water in a vacuum evaporator to give anoff-white solid residue which after air drying, weighed 146 g. These twoproducts were combined by grinding together in a mortar (weight=263.5g.). After air drying 4 days the product weight was 249 g., M.P. 90122C. Aanalysis indicated the content of to be 81.12%. Hence the productcorresponded to the structure:

The bis oxalates may be used with natural rubber and with the varioussynthetic elastomers including the styrene-butadiene rubbers, theacrylonitrile butadiene rubbers, polychloroprene, and other elastomerswhich are vulcanizable with sulfur, such as, the numerous sulfurvulcanizable dienes and their copolymers with acrylates and the like.The bis-oxalates are particularly useful also for the recently developedelastomeric terpolymers of ethylene, propylene, and a conjugated diene(the well-known EPDM elastomers; see page of Chemical & EngineeringNews, Jan. 23, 1967) which require a very high cure rate for the properdegree of vulcanization. The amount of bis-oxalate that will be employedwill generally be from about 0.1 to about 5 parts per hundred parts ofrubber (phr.), preferably from about 0.3 to 2.0 phr.

Evaluation of the oxalates in styrene-butadiene rubber Product ofExample 2=0.75 phr.(bis(dimethylarnmonium) oxalate hydrate) [Mooneyscorch at 266 F.; Cure at 302 F.; T5=16 min.]

Min 'M300 T E H Product of Example 4:0.75 phr. (bis(dietllylannnoniulu)oxalate hydrate) [Mooney scorch at 266 F.; Cure at 302 F.; T =17 min]M300 T E H TABLE I[.EV'ALUATION OF VARIOUS OXALAIESS AS is shown in thefollowing Table I which also includes ACCELERATOR ACTIVATORS IN SB R thetest recipe used and indicates in each case the ac- SBR 1606 Test Kemp 0celerator activator used. This table also shows compari- SBR 150 01 i613son of the oxalates with a control blank and with two gfbg f3 commercialaccelerator activators (Barak and Ak- Zinc oxide- 5. 0 n Stearic acid..3. 0 tone Sulfur.-. 0 MBT 1. 5

TABLE I.-EVALUATION OF PRODUCTS IN SBR 173 5 Test Recipe Blank SBR 160616 [Mooney scorch at 266 F.; Cure at 302 F.; T =24.6 min.]

M300 T E 11 Did not cure 175 375 850 425 l, 350 750 48 600 1,975 800 51925 2, 775 750 53 1, 200 3, 075 650 55 Key: M=Modulus; T=Tensilestrength (lbs./sq.); E=Elongation (percent); H= Hardness (Brinell) Barak1.0 phr. (dibutylammonium oleate) Blank [Mooney scorch at 266 F.; Cureat 302 F.; T 21.6 min] [Mooney scorch at 266 F.; Cure at 302 F.; T =20min] Min. M300 T E H M300 T E H 5. Did not cure 10 425 750 49 Did noteuro 15 1, 525 3, 400 610 56 200 1, 375 980 40 20 1, 975 3, 450 500 57775 3, 175 760 53 30 2, 225 3, 500 450 56 1, 325 3, 500 620 57 40 2, 3003, 000 410 58 1, 650 3, 450 500 60 "Aketone 1.0 phr. (a urea complex)Bamkems Pb (dibutylammonium Gleam) [Mooney scorch at 266 F.; Cure at 302F.; T 10.0 min.]

[Mooney scorch at 266 F.; Cure at 302 F.; T5=19 min.] M300 T E H M300 '1E 1, 025 2, 700 700 51 2, 025 3, 350 450 60 Did not 'cure 2,800 3,075320 63 425 250 860 2, 800 3, 025 320 61 1, 475 3, 550 550 2, 925 2, 925300 63 2, 025 3, 450 450 900 025 310 62 2, 025 3, 410

Aktone"=0.75 phr. (a urea complex) [Mooney scorch at 266 F Cure at 302F.; T =14 min.]

M300 T E H Product of Example 1 at 1.0 phr. (lanhydrousbis(dimethylammonium oxa [Mooney scorch at 266 F.; Cure at 302 F.; T514.6 min.]

Mono(d.imethylammonium)oxalate at 1.0 phr. [Mooney scorch at 266 F.;Cure at 302 F.; T5 20.8 min] M300 T E H Did not cure 1,050 2, 800 660 54l, 775 3, 350 520 58 2, 175 3, 450 490 60 2, 275 3, 425 420 62 2, 575 3,200 400 62 Bis(diphenylguanidinium)oxalate at 1.0 phr. [Mooney scorch at266 F.; Cure at 302 F.; T 19.8 min.]

M300 T E H Did not cure 650 1, 625 600 51 1, 550 2, 375 450 58 1, 775 2,350 350 60 2, 225 2, 325 310 62 2, 225 2, 325 310 61 As can be seen fromthe above Tables I and II, the bis-oxalate salts of the invention aresignificantly superior to mono dimethylammonium oxalate and also to thebis(diphenylguanidinium)oxalate, and, in fact, are superior to thecommercial agents Barak and Aktone, either in actual activator activityor in case of processing and in being less scorchy.

Evaluation of the products in EPDM-rubber is shown in Table III.

TABLE III.-EVALUATION OF BIS-OXALATES SALTS IN EPDM RUBBER Test RecipeEnjay EPT 3509 F la k Blank [Mooney scorch at 266 F.; Cure at 320 F.; T59 min.]

Min. M300 T E H Compression set (40 cure) in 70 hrs. at 212 F. 51%.

Product of Example 1 1.0 phr.

[Mooney scorch at 266 F.; Cure at 320 F.; T 7 min] Min. M300 T E HProduct of Example 2 1.0

[Mooney scorch at 266 F.; Cure at 320 F.; T5 6 Min.]

T E H Compression set (40 cure) in 70 hrs. at 212 F. 45%

Another advantage of the bis oxalate salts used in the invention is theimproved compression set values that results when they are used in EPDMelastomers. This is shown in the following Table IV which shows theresults obtained using the test recipe of Table III modified with theamounts of methyl thiram shown.

TABLE IV Methyl Thiram 1.0 plir. Product of Example 1 2.0 phr. 10[Mooney scorch at 266 F.; Cure at 320 F.; T5 5 min.]

Min M300 T E H Compression set (40 cure) in 70 hrs. at 212 F.=42%.Results with product of Example 2=2.0 phr. are similar to those givenabove.

Methyl Thiram 3.0 phr. 2O [Mooney scorch at 266 F.; Cure at 820 F.; T512 min.]

Min M300 T E H Compression set (40 cure) in 70 hrs., at 212 F.=49%.

As can be seen from the data, the compression set value Where the bisoxalate salt is used in conjunction with methyl thiram (3.0 phr. total)is significantly better than an equivalent amount of methyl thiramabove.

It will be understood that numerous changes may be made from the abovedescription and examples without departing from the spirit and scope ofthe invention.

We claim:

1. In the process of curing elastomers with sulfur, the improvementwhich comprises incorporating in said elastomer an acceleratoractivating amount of a bisoxalate of the structural formula where R ismethyl or ethyl and n is from 0 to about 3.

2. A process as in claim 1 where the bis-oxalate is anhydrous bis(dimethylammonium oxalate.

3. A process as in claim 1 Where the bis-oxalate is abis(dimethylammonium)hydrate.

4. A process as in claim 1 Where the elastomer is a styrene-butadieneelastomer and the bis-oxalate is a hydrate of his(dimethylammonium)oxalate.

5. A process as in claim 1 where the elastomer is an ethylene-propyleneterpolymer and R is methyl.

References Cited UNITED STATES PATENTS 1,698,715 1/1929 Cadwell 260-7822,131,126 9/1938 ter Horst 260-782 2,480,814 8/1949 PunshOn et a1.2607-94 3,317,918 5/1967 Foster 260-83] 3,335,118 8/1967 Kanavel et a1.260-86.1

OTHER REFERENCES Blokh, G. A., et al., Chem. Abst., 54, 10372e.

JOSEPH L. SCHOFER, Primary Examiner C. A. HENDERSON, 1a., AssistantExaminer US. Cl. X.R.

